The invention relates generally to pneumatic mechanical control systems and more particularly to a pneumatic brake and valve arrangement for use in chain hoists and similar equipment having pneumatically driven motors.
Pneumatically driven motors enjoy significant use as sources of rotary energy in diverse applications. This use is the result of many considerations, for example, output power versus size, serviceability and simplicity, to name but three. Pneumatic powered motors also exhibit insignificant spark ignition and shock hazards when compared to their electrically driven counterparts. These features and others have encouraged their acceptance on production lines, assembly stations, machine shops and service and repair facilities.
A pneumatically powered device which has found acceptance in all of these locations is a chain hoist. Typically, such devices include a relatively high speed, low torque motor, a speed reducing mechanism, a brake assembly and a hook terminated chain which is played in and out by the drive components, lowering and raising the hook. Appropriate ports in the motor housing and associated valves provide reversible motor operation and brake application when the motor is quiescent.
Hoist designs incorporating these general features are illustrated in U.S. Pat. Nos. 2,823,775, 2,927,669 and 3,125,200. The first of these patents teaches a hoist having a pneumatically powered motor and mechanically activated band-type brake. The second patent illustrates a similar device having a pneumatically activated brake. The brake comprises a fixed piston and translating cylinder and a frusto-concial brake which is supplied with air from within the motor. The last patent illustrates a hoist having a disk brake which is released by the application of compressed air to an adjacent spring-biased diaphragm. U.S. Pat. No. 3,848,716 discloses a more contemporary pneumatic hoist and brake wherein exhaust air from the motor is utilized to activate a spring-biased frusto-conical brake assembly.
Each of these designs represents a motor drive and brake configuration approached from a slightly different perspective and with slightly different weight accorded various design parameters. Similarly, each design exhibits specific shortcomings, for example, the brake in U.S. Pat. No. 2,823,775 apparently may be released without motor activation if an air supply to the hoist is lacking. The device of U.S. Pat. No. 3,125,200 is exceptionally complex and the air supply to the brake in U.S. Pat. No. 3,848,716 may create significant back pressure in the motor exhaust thereby reducing the efficiency and power output of the motor.